This invention relates to a multi-layer coated metal tube, and more particularly to a metal tube having a primed outer surface coated with a plurality of layers of plastic material securely bonded thereto.
Metal pipes often have their outer surfaces covered with a protective coating. These pipes are used for conveying hydraulic fluids (such as for brakes, transmissions, power steering, fans, and the like), fuels (such as methanol, ethanol, gasoline, additives, diesel, kerosene, jet fuel, and mixtures thereof), vapor, and the like in a motor vehicle. As such, these pipe lines are located under the body of the vehicle. Since they are used in such a harsh environment, the pipes are required to have a high degree of corrosion resistance, scratch resistance, impact strength and mechanical wear resistance. In cold climates, it is not unusual to encounter road salt sprinkled onto road surfaces in order to prevent freezing of water on the road surfaces and the inherent dangers caused thereby. The popularity of spreading road salt has created a serious problem of pipe corrosion. The pipes are also vulnerable to damage or wear from stones or mud spattered by rotating wheels of the vehicle. It is, therefore, necessary that the pipes attached to the underbody of the vehicle be coated so as to resist both chemical corrosion and mechanical damage or wear.
Flexible fuel vehicles (FFVs) often are designed to be fueled by methanol, ethanol, gasoline, and/or any combination of these fuels. Certain fuel blends may corrosively attack metals, even some types of stainless steel. If corrosion is prolonged, fuel and/or hazardous fluids may leak from the tubing. However, emissions standards, for example those issued by the California Air Resources Board (CARB) and the U.S. EPA, mandate no leakage or permeation for as high as 150,000 miles or 15 years.
A double-rolled steel pipe has been proposed that is made by rolling a steel strip or hoop twice and brazing its longitudinal edges by means of a copper plating layer, or a seam welded steel pipe, where the pipe has an outer surface coated with an electroplated zinc film. The zinc film has an outer surface coated with a relatively thin special chromate film having, for example, an olive color. Chromate is typically used to enhance the adhesion of the subsequent layer by making the surface rougher. The chromate film has an outer surface coated with a fluorinated resin film. The fluorinated resin film is formed by impregnating the chromate film with a dispersion of polyvinyl fluoride immediately after the formation of the chromate film when it is still in the state of a gel, and drying them under heat, so that the fluorinated resin film may form an intimate bond with the chromate film. When the chromate film is formed by treating the pipe with a solution, it requires large amounts of a chromium compound and an organic acid, such as formic acid, used as a reducing agent. It is necessary to supply the treating solution with the chromium compound frequently, and to renew it at regular intervals of time in order to maintain a constant film forming capacity. The waste solution, however, contains a large amount of chromium having a valence of 6, which is a toxic substance. As such, the disposal of this toxic waste solution is very costly. Although the chromate film as formed is highly resistant to corrosion, the heat to which it is exposed during the formation of the resin film deprives it of water and thereby makes it brittle. Any plastic deformation of the pipe, such as may result from bending or double flaring, and/or any elastic deformation, such as may result from pipe handling prior to or during assembly, may form fine cracks in the chromate film which lowers its rustproofing properties.
It has also been proposed to provide a corrosion resistant pipe where a metal pipe is provided with an outer surface coated with a zinc film, a chromate film, an intermediate layer consisting sequentially of an epoxy resin, and a polyvinyl fluoride film formed one on top of another in the order listed.
A plastic-coated steel tube has also been proposed where a steel tube has an inner layer of at least one cross-linked polyolefin modified with a hydrolyzable silane and an outer unmodified or soot-blended polyolefin layer on the exposed surface of the inner layer.
A process for coating metal tubes with plastic material has also been disclosed where a fixed metal tube is heated to a temperature above the melting point of the plastic material to be employed, thereafter causing a mixture of plastic powder and air to pass through the metal tube whereby the plastic material is fritted onto the inside surface of the tube, thereafter rotating the metal tube and applying to the exterior surface thereof in a plurality of stages a plastic material, the plastic material being electrostatically sprayed onto the rotating metal tube. After each stage of electrostatically applying plastic to the outside surface of the metal tube and applying plastic material to the inside surface thereof, the plastic material is completely melted and smoothed. However, this process has several drawbacks, including that it is a batch process (as opposed to a continuous process); it requires rotation of the tube; and the cure time of the plastic material is quite slow, eg. several minutes. Thus, this process is rather inefficient, both from a cost and time standpoint.
An automobile tube line for a brake, fuel or hydraulic system has also been disclosed with an interior steel tube having a galvanized exterior layer with an additional exterior olive chromated layer which is wrapped in an additional Nylon 12 layer casing where the plastic casing is a polyamide layer applied by extrusion on top of the olive chromated layer.
In U.S. Pat. No. 5,590,691, which is incorporated herein by reference, there is disclosed a metal tube having an extruded multiple plastic layer coating bonded thereto. This tube is very corrosion resistant, yet may not be suitable in aggressively corrosive environments such as for example upon exposure to fluid soaking in 85% ethanol.
Thus, it is an object of the present invention to provide a corrosion resistant metal tube which withstands prolonged exposure to corrosive environments, including aggressively corrosive environments. It is another object of the present invention to provide such a tube and process for making the same which advantageously substantially contains any volatile organic compounds, thereby substantially preventing any undesirous venting of VOCs to the atmosphere. It is a further object of the present invention to provide a process which captures and re-uses any primer material applied to the tube, thereby advantageously substantially eliminating waste of raw material(s) and substantially preventing undesirous venting/overspray of primer to the surrounding environment. Still further, it is an object of the present invention to provide such a tube and process for making the same which exhibits good adherence between the tube and a subsequent corrosion resistant layer(s) for prolonged periods of time. Further, it is an object of the present invention to provide a process for making such a tube which-is high speed thereby being advantageously cost effective.